(1) Field of the Invention
The present invention relates to the field measuring the mechanical response of micro-electro-mechanical systems. More specifically, the present invention pertains to a method and apparatus that allows the direct control of the load applied to a micro-electro-mechanical system in order to measure the mechanical response.
(2) Description of Related Art
The interest in micro-electro-mechanical systems (MEMS) has given rise to a necessity for developing new techniques for studying the mechanical response of structures with small dimensions, such as thin, free-standing films, membranes, and cantilevers. The trend to reduce the dimensions of these structures compels load sensitivity in the sub-milli-Newton scale at displacements in the sub-micrometer range, and the interest in characterizing active materials, such as ferroelectrics, ferromagnetics, and shape-memory alloys, requires dynamic measurements under load control. These requirements are not met by conventional techniques.
Previously, tensile tests have been used to determine the mechanical properties of structural materials. The American Society for Testing and Materials (ASTM) dictates a tensile test as the standard for determining the mechanical properties of structural materials. While uniaxial and uniform stress and stain fields can be measured directly using tensile tests, at the micrometer scale tensile tests are difficult and expensive.
Another conventional technique is the bulge test. In the case of the bulge test, a membrane is supported at its edges, and the response is a plot of deflection verses applied pressure.
There are other well-established devices that are commercially available for performing mechanical characterization. One is the mechanical testing system, such as MTS or Instron. The mechanical testing system works with a large load, having sub-Newton sensitivity and large displacement while providing sub-micrometer resolution. Another type of device for performing mechanical characterization is a conventional atomic force microscopy (AFM), working in the nano-indentation mode. AFM works with very gentle load, ranging from 1-100 nano-Newtons (nN), and fine displacement, ranging from 1-1000 nanometers. One drawback is that using both MTS and AFM, there is a big gap between the measurements that can be taken, both in terms of force and displacement. Unfortunately, this gap is the exactly the region that MEMS devices operate in. Recently, many efforts have tried to expand these two well-established methods into this region. For example, efforts have included changing cantilevers and tips and replacing positioners of the AFM or replacing driving motors with piezo-actuators and changing the loading frame and load cell of the MTS. However these efforts have had limited success. Therefore, there is a need for a completely new technique for studying the mechanical response of structures with small dimensions.
In addition, both the MTS and AFM (when in nano-indentation mode) are inherently displacement controlled: i.e., they impose a fixed displacement and measure the load. However, given the fragile nature of MEMS structures and the nonlinear features of active materials, it is possible to have sudden load increases and failure under displacement control. Therefore, it is desirable to test fragile materials, such as MEMS structures, in a load-control device (i.e., a device that imposes a force and measures the displacement). The conventional MTS and the AFM (when in nano-indentation mode) can be adapted for load control by means of a feedback loop. But there are severe limits on the response time, and this is effective only for quasi-static testing, and not for dynamic testing. Further, the resulting transients from displacement control can lead to large undesired loads in such devices. Therefore, there is a real need for a new technique that is inherently load controlled.